Solar panels (such as thin film solar substrates) generate an electrical current in the active area of the panel where the active area includes an absorbing layer that absorbs light energy and converts it into electrical energy. An exemplary absorbing layer includes thin layers of material comprising Cu(In,[Ga])Se (=CI[G]S) which are known to exhibit a high photovoltaic conversion efficiency. As used herein, the term “CIGS” refers to a family of cells using similar absorbing materials, but in somewhat different compositions, etc. The CIGS family includes at least the CIS (Copper-Indium-Sulfide or Copper-Indium-Selenide), CIGS (Copper-Indium-Gallium-Selenide) and CIGSSe (Copper-Indium-Gallium-Sulfur-Selenide) type solar substrates.
The current generated in the active area is transferred to an external electrical circuit. For this purpose, conductors, sometimes referred to as busbars (i.e., one for the positive electrode and one for the negative electrode), may be connected along edges of such a solar panel to collect the generated current. Often the conductors are glued using a conductive epoxy or the like, or soldered by thermal or ultrasonic soldering; however, process limitations and cost have resulted in other interconnection technologies being considered instead of gluing or soldering. FIGS. 1A-1B illustrate the use of conductive ribbons providing the interconnection. Referring to FIG. 1A, solar substrate 100 (i.e., a CIGS type thin film solar substrate) includes glass base material 102 (or another base material such as steel, metal foils, polyimide, other lighter or flexible base materials, among others) and absorbing layer 106. Absorbing layer 106 includes the active material that absorbs and converts light energy into electrical energy. Conductive layer 104 is provided between glass base material 102 and absorbing layer 106. Conductive ribbons 108a and 108b are ultrasonically bonded to electrode regions 104a, 104b at respective bonded portions 108a1 and 108b1. It is important that the bonded portions be of sufficiently low contact resistance to not adversely affect the panel's efficiency, and be very reliable over the life time of the panels. FIG. 1B is a cross-sectional view of solar substrate 100 illustrating a current flow “i” from conductive ribbon 108a to conductive ribbon 108b. As shown in FIG. 1B, absorbing layer 106 includes an upper transparent conductive oxide (TCO) layer 106a (e.g., where the TCO layer may be ZnO or the like for a CIGS type solar substrate).
In an exemplary CIGS solar substrate the absorbing layer initially covers the electrode regions of the conductive layer. Thus, the absorbing layer is removed from the electrode region prior to the ultrasonic bonding of a conductive ribbon or the like. The bulk removal of the absorbing layer in the electrode region may be accomplished using various processes such as scraping and brushing. Unfortunately, contamination of the electrode region still present after removal of the absorbing layer results in challenges to ultrasonic bonding. Such contamination can be caused by an interaction layer between the absorbing layer and the electrode layer.
Thus, it would be desirable to provide improved systems and methods of processing solar substrates for ultrasonic bonding and the like.